The so-called chipping is known, which is a phenomenon that scratches are produced on a coating film applied onto a body outer plate and exterior parts of automobiles by collision of flying stones and gravels during traveling. For a purpose of protecting the coating film and a substrate from this chipping phenomenon, for example, in a floor rear portion of an automobile, a coating made of a vinyl chloride plastic sol composition has been conventionally applied to a steel sheet surface subjected to cathodic electrodeposition coating. However, the vinyl chloride plastic sol composition containing polyvinyl chloride may produce hydrogen chloride gas in an incineration treatment process of steel plates of waste automobiles, and may damage an incinerator.
Moreover, other than the floor rear portion, for example, in the side surface of the automobile body or the like, usually, an intermediate coating having chipping resistance is applied onto a steel sheet surface subjected to cathodic electrodeposition coating. As such an intermediate coating, a coating mainly containing a polyester resin or a polyurethane resin and using an organic solvent as a solvent is usually used. However, the coating using an organic solvent is preferable from the viewpoint of stability and operability of the coating, but is not preferable from the viewpoint of environmental impact (VOC and the like) and therefore, improvement is demanded.
Under such circumstances, in recent years, a composition for an aqueous coating has been proposed as a coating instead of the vinyl chloride plastic sol composition and the organic solvent coating.
However, the composition for an aqueous coating uses water as a dispersion medium. For this reason, there is a problem that it is easy to produce blisters of the coating attributed to vaporization of moisture particularly upon drying at a high temperature (approximately 90 to 160° C.). The blisters of the coating film are more easily produced as the film thickness of the coating is thicker. This problem is particularly serious in the case where a coating film having a thick film thickness needs to be formed in a specific part of the floor rear portion of the automobile.
From the viewpoint of energy saving by reducing the weight of vehicles, the market needs a coating film having a thinner film thickness. However, reduction in the film thickness leads to a problem of reduced chipping resistance.
Moreover, in a coating line for automobiles, usually, a coating composition is heated and dried with a heat drying furnace after about ten minutes to several hours after the coating composition is applied. However, in the case where an automobile coated under an environment at a high temperature and high humidity such as a rainy season and summer immediately before a holiday or before a factory enters long holiday intermission is left under an environment at a high temperature and high humidity for several days to around 10 days before heating and drying, coating film defects such as blisters and cracks may be produced when the coating of the automobile is heated and dried after the automobile is left as it is.
In order to solve such problems that the composition for an aqueous coating has, many related arts have been disclosed. For example, a copolymer latex (Patent Document 1) has been proposed in which various performances are improved by devising a method for adding a raw material monomer. Moreover, a copolymer latex containing a raw material monomer such as an epoxy group has been proposed (Patent Document 2).
Further, with respect to different layer structure type latices represented by a core shell latex, various latices have been proposed although description about an aqueous baking coating is not observed (Patent Document 3).